Fingerprint Detection Experiment

To detect and obtain fingerprint patterns from our fingers

Forensic and criminal investigations

Basic theory
Fingerprints are a unique identity that differs from one person to another. This is a pattern, or trace of the friction region, or lifting the epidermis on the palm of the hand of the finger. The print can be stored in natural hand secretions, or palm sweat, and found on the side of the palm of the finger. The identification of these fingerprints is an important system in the police institution, since taking fingerprints from crime scenes can be used in forensic science to identify suspects or others involved in crime, or to recognize whether two fingerprints come from the same finger or palm.

Fingerprint Detection Experiment
Latent Fingerprint
Three types of prints are commonly used in fingerprint identification, plastics, visible fingerprints, and latent fingerprints typically used in crime scenes. Plastic molds are trace areas of soft material such as wax and visible fingerprints are visible traces of colored materials. Both types are directly visible to the eye without processing. Only an invisible fingerprint (latent fingerprint), due to the sweat of the body that is in the finger region to the surface of the object, requires a technique to visualize the print.

Fingerprint Detection Experiment
Plastic fingerprint

Fingerprints on surfaces that are not absorbed by the skin or the skin of the back of the palm can be developed with powder, while molds on soft or porous surfaces require visualization using chemical processing techniques.

Fingerprint Detection Experiment
Visible fingerprint
One of the chemical techniques used to visualize fingerprints is Iodine vapor. When the finger region is in contact with the surface of the print receiver, the material in the palm of the hand like oil, will be transferred to the surface. Patterns on paper may differ depending on the elasticity of the skin, the pressure applied by the finger, and the smoothness of the paper.

Therefore, when the finger is pressed against white paper, a different pattern will be obtained from the prints left on paper. At this time, iodine can be heated to produce purple vapor which is then absorbed by fingerprint secretion to produce a yellowish brown pattern on the surface where the secretions are stored. The visualization is short-lived and needs to be chemically determined by mixing with flour.

Tools and materials
Solid Iodine crystals
White paper
Glass tube
Clamp clamp
Tweezers or Spoon
Starch solution
Stirring bar

Clamps and statives are recommended because they help to improve the position of the tubes to be heated. However, if the city has other methods to heat the solid iodine without our hands directly touch the tube, these materials are not needed. Similarly, bunsen and tube are not needed if you have a safer alternative method to heat the iodine crystals.

Iodine crystals
Iodine crystals

Iodine fumes are toxic and corrosive. Therefore, special care must be taken to prevent direct contact between the face and the smoke. Protective devices, such as nose covers and gloves, are usually used in laboratories. For the same reason, it is advisable to look directly into the tube to prevent inhaling smoke every iodine into steam. Also, it is better to use a tube with a lid, where the tube will become a confined space during heating, and it is still feasible to insert the white paper into the tube after evaporation of iodine.

1. The first step is to evaporate iodine on a tube with a cover, on a tube that is washed and dried

Fingerprint Detection Experiment
Dry tube

2. A small iodine crystals taken from the container using a sterilized spoon or tweezers, weigh the iodine.

3. We make sure the amount of iodine is small, because the vigorous evaporation force can push the lid out of the test tube

Fingerprint Detection Experiment
Iodine crystals in spoon

4. To ensure the tube will not be damaged by dropping the crystal, solid iodine is inserted along the side to the bottom of the tube while holding the tube in the corner.

Fingerprint Detection Experiment
Slipped the Iodine crystals in the test tube

5. The tube is clamped up and fixed the position of the tube to ensure the bottom of the tube will be exposed to the exterior of the fire. Insert the cap into the tube. Place the bunsen under the tube and cover it with a flame.

Fingerprint Detection Experiment
The position of the tube against bunsen

6. Heat down of the tube, where the iodine is, for a short time until the purple iodine vapor spreads throughout the tube, close bunsen. Now the black iodine deposits appear on the side of the tube ..

Fingerprint Detection Experiment
Iodine evaporation

7. After the iodine sublimes all the tubes, secretions or unsaturated fatty acids in the rest of the fingerprints will be transferred to a porous surface, or a white paper

Fingerprint Detection Experiment
Sublimation of iodine

8. Make sure there are no stains or fingerprints on the paper before being cut. Cut a large piece of paper and cut it into the following sections to put in a tube

Fingerprint Detection Experiment
Paper cut

Fingerprint Detection Experiment
Paper Slide

9. Place your thumb completely onto the paper to get as much of the fingerprint of your finger.

Fingerprint Detection Experiment
Pressing hands

10. Because slightly different ways of compressing fingers can result in different visualizations of fingerprints on paper, repeat the same process on at least three paper slides to ensure the quality of the results. Remember how hard you press your finger according to each paper slide. Once the secretions are obtained, put the white paper into the tube and quickly close the lid.

Fingerprint Detection Experiment
Putting paper on a tube

11. Opening the lid for a long time will let iodine vapor flow out of the tube. Placing the tube in front of a clearer black background indicates the yellowish color of the fingerprints after absorbing iodine vapor. At this time, the fingerprint must be clearly indicated on the paper. Paper taken out of the tube.

Fingerprint Detection Experiment
Paper that has been removed from the tube

12. Repeat the same process for all paper slides that contain fingerprints. Compare the results.

Fingerprint Detection Experiment
Compare results with multiple attempts

13. Further steps can be taken by dipping the glass rod into a starch solution then dispersing the starch solution using the stem onto the paper to fix the fingerprint patterns on the paper permanently. Otherwise iodine will evaporate into the air and the fingerprint pattern will be lost from the paper.

Fingerprint Detection Experiment
Fingerprint determination

Based on whether you push very hard on paper and fingerprints remain on paper, qualitatively, does the force of emphasis affect the clarity of the fingerprint obtained

Does the fingerprint manage to appear on the paper after being placed into the tube and after being fixed by the starch solution? Is there another way to get clear figureprints on paper? Does your fingerprint look exactly the same when fingerprints are obtained by compressing the paper using the same finger for several times?

Other parameters in the experiment can be checked as well. For example, tubes of different volumes may be applied in different tests to see if smaller volumes of tubes can condense iodine vapor inside the tube and provide a clearer pattern of fingerprints after the experiment. In a similar way whether different amounts of iodine can also be tried with multiple trials, as well as different locations you place fingerprints inside the tube.

Not only iodine can be used in visualizing fingerprints, the same material can be used, for example smoky cyanoacrylate that forms a white complex with salt in figureprints. Similarly, silver nitrate that reacts with sodium chloride in the palm sweat and forms a complex, can be heated for smoke to visualize fingerprints using the same material

The complex needs such as being visualized under UV light, which is not accessible everywhere. In addition, silver is a heavy metal and much more expensive to buy than iodine. Therefore, iodine is considered a more convenient and realistic method to use.

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